Conventional Jackup Drilling units are well known in the oil and gas industry as a solution for drilling in shallow water. Smaller Jackups often use tubular structures as legs whereas larger jackups tend to use truss legs consisting of chords, interconnected with braces. The Jackup concept and the issues discussed here would apply equally to both drilling and production units.
Conventional jackup units are intended only for operation in climates subject to little or no sea ice. While tubular legs could theoretically be sized up such that the shell is able to resist local ice pressures, it would be difficult to be incorporated with powerful leg jacking and holding systems that would be required for the large weights and ice loads expected. A conventional truss leg on the other hand is very good at transferring global loads and incorporating a powerful leg jacking and holding system, however the leg members are not designed for local loading from ice and could only resist reasonably small ice loads. Simply scaling the brace members could increase the local member strength but the increase could not be significant enough to make this an attractive option. The truss leg has an additional disadvantage that ice may accumulate inside the leg, resulting in increased ice loads. It has also been suggested that Jackup legs could be protected by the attachment of additional structures such as cones or plating. These additional structures may have to be installed on site and are likely very heavy, resulting in significant installation costs.
Structures presently used in areas subject to sea ice are generally large stiffened plate structures, consisting of shell plating, supported by a grillage of stiffeners and girders. These structures are effective in resisting ice loads, but are very large and not usually able to be moved easily from one location to another.